1. Field of the Invention
The present invention relates to an electron gun of an electron microscope or electronic drawing apparatus, particularly to downsizing of an electron gun.
2. Discussion of Background
A conventional scanning electron microscope (SEM) or electron-beam drawing apparatus (EB) accelerates an electron beam emitted from an electron gun constituted by a field-emission or thermal-field-emission electron source, forms the electron beam into a thin electron beam by an electron lens, forms the thin electron beam into a primary electron beam, thereby scans the surface of a sample with the primary electron beam by an electronic deflector to obtain an image by detecting obtained secondary electrons or reflected electrons when the SEM is used or to draw a previously entered pattern on a resist film applied onto the sample when the EB is used. The material of the electron source uses tungsten when a general-purpose SEM is used. Moreover, an electron source for semiconductor may use a material obtained by adding zirconium to tungsten. Furthermore, LaB6 may be used for the EB.
To emit a preferable electron beam from the above electron source for a long time, it is necessary to keep the circumference of an electron source at a high vacuum pressure (10−9 to 10−10 Torr). Therefore, a method has been used so far in which the circumference of an electron gun 16 is forcibly exhausted by an ion pump 13, as shown in FIG. 2. The ion pump 13 has an advantage in that it is possible to keep a pressure of 10−10 Torr or lower only by current-carrying because the pump has no movable part. However, because the ion pump 13 has a size of tens of centimeters square or more and generates a magnetic field, the pump 13 requires a considerable volume because a magnetic shield 15 is necessary for the electron gun side.
Paragraph 0033 of JP-A No. 149850/2000 discloses a charged particle beam apparatus having a built-in getter ion pump in a lens tube as means for downsizing an electronic optical system. Moreover, a charged particle beam apparatus having a built-in nonevaporative getter pump in an electron gun chamber is disclosed in FIG. 3 of U.S. Pat. No. 4,833,362; Paragraph 0033 of JP-A No. 149850/2000; and FIG. 2 of JP-A No. 111745/1994. The getter pump mentioned above means a vacuum pump for activating and evaporizing a getter by heating it and adsorbing impurities into the getter. Moreover, the nonevaporative getter pump denotes a vacuum pump constituted by using an alloy for adsorbing gas by only heating a getter without evaporizing it. From the viewpoint of downsizing, it is more preferable to use the nonevaporative getter pump.
Furthermore, Paragraph 0014 of JP-A No. 294182/2000 discloses an electron gun in which an axis adjustment screw for adjusting the axis of an electron source is installed on the circumference of a flange. Further, JP-A No. 188294/1994 discloses a charged particle apparatus having a differential exhausting structure for keeping the circumference of an electron source at an ultrahigh vacuum pressure in its FIG. 9. Further, JP-A No. 325912/2001 discloses a technique for improving the exhausting efficiency of a vacuum chamber by making a hydrocarbon-based gas remaining in a sample chamber react with active oxygen introduced into the sample chamber and thereby decomposing the gas and active oxygen into water and carbon dioxide, which are easily exhausted.